1. Field of the Invention
The present invention relates to a backlight module control system, and more particularly, to a light-emitting diode (LED) backlight module control system and a control method thereof.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a prior art backlight module control system 100. The backlight module control system 100 includes a driving circuit 110, an LED module 120, a feedback compensation circuit 130 and a DC/DC converter 140. In addition, the driving circuit 110 includes a plurality of current sources 112 and a plurality of switches 114 respectively corresponding to the current sources 112. The LED module 120 includes a plurality of LED sub-modules 122 where each LED sub-module 122 includes a plurality of LEDs connected in series. With reference to FIG. 1, operations of the backlight module control system 100 are described as follows:
First, the switches 114 are selectively turned on or off according to a pulse width modulation (PWM) signal to generate a driving signal, and the LED sub-modules 122 are enabled or disabled according to the driving signal. When the switches 114 are turned on (i.e., the LED sub-modules 122 are enabled), the feedback compensation circuit 130 gets voltage values of nodes Vm1, Vm2, Vm3, . . . , Vmn and provides a compensation value to the DC/DC converter 140. Then, the DC/DC converter 140 outputs an operating voltage VLED required by the backlight module control system 100 according to the compensation value.
When the switches 114 are turned off (i.e., the LED sub-modules 122 are disabled), because a capacitance of each LED sub-module 122 is greater than a capacitance between each node (Vm1, Vm2, Vm3, . . . , Vmn) and ground, the voltage levels of the nodes Vm1, Vm2, Vm3, . . . , Vmn approach the operating voltage VLED.
In general, current controls of the current sources 112 are implemented by current sinks. In other words, a current of each LED sub-module 122 is controlled by current mirrors of the driving circuit 110. In addition, in order to lower the power consumption and increase the driving ability, most of the driving circuits using the current sinks have lower withstand voltages (about 60 volts). As described above, the voltage levels of the nodes Vm1, Vm2, Vm3, . . . , Vmn approach the operating voltage VLED when the LED sub-modules 122 are disabled, therefore, the operating voltage VLED cannot be designed to be greater than the withstand voltage of the driving circuit 110. Therefore, a quantity of the LEDs included in each LED sub-module 122 is limited. For a large size display panel requiring many LEDs, more driving circuits 100 of the backlight module are needed and the cost is thereby increased.